client IPv6 support [BEP-7]

Depends on the next versions of the following modules:
* string2compat
* compat2string
* addr-to-ip-port

You can test with torrents from Bitlove.org, the tracker always returns the IPv4 & IPv6 address of the prittorrent seeder.
I was adding some tolerance in #40, and because the non-compact format essentially allows free-form hostnames, why not check if this is an IPv6 address at all?

It's not important to me and might lead interoperating people to assume a false level of flexibility in the HTTP tracker protocol.